As a two component developer material employed in the development method in copiers, laser printers, and the like, utilizing the electrophotographic system, a negatively charged toner is generally mixed with a positively charged carrier. The carrier is employed to apply, to the toner, an appropriate amount of triboelectrical charge with negative polarity.
In recent years, image forming apparatuses such as laser printers, to which the electrophotographic system is applied, have shown a tendency toward a decrease in the size. Along with this tendency, the image forming apparatus itself and particularly, the development device comprising a development unit have been subjected to the decrease in the size. Accordingly, in a small-sized development device, the amount of the developer material employed to develop electrostatic latent images inevitably becomes small.
In the above-mentioned negatively chargeable two component development material, during a short period of time while a supplied toner is transported to a development zone to develop an electrostatic latent image, an appropriate amount of negative polarity triboelectrical charge is required to apply to the above-mentioned toner, that is, improvement in charge rising properties is required.
In view of the foregoing, as a means to improve the charge rising properties, a technique is disclosed (for example, in Japanese Patent Publication Open to Public Inspection No. 2-8860) in that a positively chargeable charge control agent is incorporated into the resin coated layer of a resin coated carrier.
Positively chargeable charge control agents known in the art include quaternary ammonium compounds disclosed in Japanese Patent Publication Open to Public Inspection Nos. 49-51951, 52-10141, and alkylpyridinium compounds and alkylpicoridium compounds (for example, nigrosine SO, nigrosine EX, etc.), disclosed in Japanese Patent Publication Open to Public Inspection Nos. 56-11461 and 54-158932.
The conventional positively chargeable charge control agents known are organic compounds having a large cohesive force and are inferior in being dispersed to and mixed with a coating resin. Due to that, it has been impossible to uniformly disperse the positively chargeable charge control agent into the resin coating layer of the carrier and to apply, to the toner, an appropriate amount of the triboroelectric charge with negative polarity. As a result, there are problems with toner scattering and background stain.
On the other hand, in a magnetic carrier core constituting the carrier employed in the two component developer materials, the ferrite series carrier core comprising heavy metals such as copper (Cu), etc. has been widely employed. However, from the view of the adaptability to the recent environmental regulations, carrier cores require no such heavy metals. In this case, a so-called magnetite composition core (magnetite core) exhibits remarkably high adaptability in which the carrier core is only composed of iron (Fe), and oxygen (O), and has a crystal structure in that ferrous oxide (FeO), and ferric oxide (Fe.sub.2 O.sub.3), are mixed as constituting components.
The magnetite composition core shows a tendency to have comparatively large magnitudes of magnetism and is not preferred for a development method in which an electrostatic latent image is developed in a non-contact state. Accordingly, it has been difficult to apply the magnetite core to a non-contact development method unless the magnitude of magnetism is adjusted to the decreasing direction without varying the magnetite composition.
During an initial stage, and even normal running stage, carrier adhesion results in abrasion on an electrostatic latent image bearing body and causes a problem of white streak image defect.